In a Public Land Mobile Network (PLMN), a Service Aware (SA) node such as the so-called Flexi ISN can be deployed behind a Gateway GPRS Support Node (GGSN), corresponding to a so-called Gi-deployment, to enable operators to offer services in a more flexible and efficient way. The communication between GGSN and SA node is performed through the Network Access Server (NAS) interface. NAS interface uses Radius Authentication Dial In User Service (RADIUS) authentication for allocating the IP address (if needed) and RADIUS accounting for providing the IP address, International Mobile Subscriber Identity (IMSI) and Mobile Subscriber ISDN (MSISDN) to the SA node. The NAS can be for example a 3rd party GGSN that acts as a RADIUS client and a SA node that acts as a RADIUS server. The user plane traffic is carried in plain IP, Generic Routing Encapsulation (GRE) or IP-in-IP tunnels and the RADIUS signalling is used to set up the user plane traffic in a similar way as in the GPRS/3G networks. The NAS interface can be referred to as being the southbound Gi interface, as an alternative to the Gn interface.
During the Public Data Network (PDP) Context activation procedure, an “Accounting-Request-START” message, including an Access Point Name (APN), is sent from GGSN towards the SA node to inform it about the subscriber identification (IMSI, MSISDN) so that user profile can be determined. Upon receiving of the message a NAS context is established. Thus, the START message is used to inform SA node about new sessions, so that SA node can monitor them properly. An “Accounting-Request-STOP” message can be used by GGSN to inform the SA node about a possible PDP Context termination, thus SA has to delete the already established NAS Context.
RADIUS Accounting-Request-Start and Stop messages may be used during both primary and secondary PDP context activation and deactivation procedures, respectively. If the SA node is used for IP address assignment, then, upon reception of a RADIUS “Accounting-Request-STOP” message for all PDP Contexts associated to a session, the SA node (server) may make the associated IP address available for assignment.
In order to avoid race conditions, the GGSN shall include a 3GPP Vendor-Specific sub-attribute Session Stop indicator when it sends the “Accounting-Request STOP” for the last PDP context of a PDP session and the PDP session is terminated (i.e. the IP address and all GTP tunnels can be released). The SA server shall not assume the PDP session terminated until an “Accounting-Request STOP” with the Session Stop Indicator is received. In 3GPP reference architecture, the SA function corresponds to Flow Based Charging (FBC), which is implemented in Release 6 TPF (Traffic Plane Function) and in Release 7 PCEF (Policy and Charging Enforcement Function). 3GPP does not define interface between GGSN and TPF/PCEF. SA function can be integrated as part of GGSN or it can be implemented separate node connected to GGSN via Gi interface defined in 3GPP specification 29.061, which corresponds to the NAS interface described above.
Even though 3GPP reference architecture does not specify details about the case where GGSN and TPF/PCEF are implemented as separate nodes, this alternative is often used in practice. For operators, this approach has the benefit that SA can be introduced without making changes to existing GGSN nodes.
However, there may be a need for a method of locating an NAS context, which improved performance; in particular, with respect to a more reliable or fail-proof locating of the NAS context in order to enable an improved release of the IP address and/or all GTP tunnels.